The so-called sintering method which uses a powder as a starting material and converts it into a mass by sintering has heretofore been widely used on metals, ceramics, and composite materials thereof.
The largest task imposed on the sintering method consists in obtaining a mass which is compact and is formed of minute crystal grains. For promoting compaction, pressure sintering methods resorting as to a hot press and a hot hydrostatic press are available. They use pressures on the order of 500 atmospheres and 2,000 atmospheres respectively. For the heating to be made under a still higher pressure, it is necessary to use such an apparatus for the generation of ultrahigh pressure and high temperature as disclosed in Japanese Patent Publication SHO 36(1961)-23,463, for example. With this apparatus, sintering can be carried out under not less than 10,000 atmospheres of pressure.
For example, a method which obtains a sintered article having a compact and fine texture by sintering an alloy powder or a nonequilibrium phase under not less than 10,000 atmospheres of pressure is conceivable.
The problems encountered by the ultrahigh pressure sintering method are as follows:
1. Since this method uses a solid pressure medium, the apparatus is deficient in capacity for hydrostatic pressure and the sintered article assumes anisotropy.
2. Since a heater installed outside the material under treatment or resistance heating caused by passage of electricity to the material under treatment itself is utilized for the heating, the apparatus requires a power source of a large capacity.
3. Since no suitable substance is available for an electrically insulating refractory material, the heating at an elevated temperature exceeding 2,000.degree. C. is not easily attained.
4. Since the speed of heating is slow, diffusion occurs and crystal grains are coarsened during the course of temperature elevation.